1. Field of the Invention
This invention relates to large relative aperture objective lenses, and, more particularly, to objective lenses of increased relative aperture while still permitting high grade imaging performance throughout an extended focusing range from infinity to very short object distances.
2. Description of the Prior Art
In the past, the photographic objectives called macro, or micro lenses which are adapted to be mainly used in closeup shooting and also capable of focusing up to infinity, when focused down to close object distances have suffered darkening of the effective F-number. This led to the production of a shade in the split prism area on the focusing screen. Therefore, the photographer had to turn to the surrounding or mat area for focusing purposes. Also, since the depth of field became shallow, it became very difficult to detect when the objective lens is focused to an optimum in-focus position. For this reason, there is an increasing demand for a larger relative aperture or fast objective lens.
However, since the range of variation of the image magnification is wide as compared with the normal photographic objective, aberrations vary to large degree with focusing. This aberration variation becomes prominent when the relative aperture is increased. Even in case, as usual, the start point for design on the image magnification is set in about 1/10 times, it is found that as the image magnification approaches 1/5 times or larger, the spherical aberration and curvature of the field becomes under-corrected, and asymmetric aberrations for off-axis rays are produced. Particularly regarding the outward coma in the intermediate zone to the marginal zone of the image format, for a subject of fine patterns, the image is largely blurred to impair the degree of sharpness, thus the photographer is obliged to use the objective with the diaphragm closed down. As it is also required to remove the distortion, it follows from these points that the symmetric lens type, for example, Gauss type with the inclusion of the modified Gauss type is suited for macro lenses of large relative aperture, and has found many uses.
With the ordinary Gauss type lens, however, in closeup shooting, the paraxial ray becomes stronger in divergence than when focused at infinity so that the height of incidence on the rear lens component becomes higher. The result of this is that the spherical aberration is largely under-corrected by the positive refractive power of the rear component. On the other hand, as regards the off-axis coma, because focusing down to shorter object distances is effected by moving the lens system forwards as a whole, the principal ray to the maximum image height makes a smaller angle with the optical axis so that the upper and lower portions of the image bearing beam are refracted more weakly by the rear lens component than when in shooting at an infinitely distant object, thereby outward coma is produced.
As a means for compensating for this drawback, it has already been known in the art, as disclosed in Japanese Laid Open Patent Sho Nos. 50-138823, 52-7723, 53-87728 and 53-10425 and U.S. Pat. Nos. 3,884,557 and 4,260,223, to change a certain air separation between lens members, as the entire lens system is moved forward, away from the image plane, with some advantage. However, this method, though being favorable to the closeup shooting of low image magnification, has drawbacks that, as the image magnification increases, the state of correction of the image aberrations cannot be maintained sufficiently acceptable. Further while the off-axis asymmetric aberrations are well corrected, the other various aberrations are somewhat sacrificed. The problem of achieving a large increase in the relative aperture while maintaining good stability of aberration correction throughout the extended focusing range has not been solved yet.